Determination of position

Abstract: An autocalibration system includes two loudspeakers (2, 4) spaced from three microphones (16, 18, 20). Acoustic transient pulses are emitted by each loudspeaker (2, 4) in turn and from the reception of of these pulses by each of the microphones (16, 18, 20), the time-of-flight for each pulse to each microphone may be derived. From these time-of-flight measurements are also derived the distance and angular displacement of each microphone (16, 18, 20) from a reference point (12) flanked by the loudspeakers (2, 4).

Description: The present invention relates to a method and apparatus for determination of position and has particular, although not exclusive, relevance to use in so-called dummy-head recording techniques. An example of a dummy-head recording system is disclosed in United States Patent US-A-4,119,798. In this document a dummy-head having microphones mounted in the ear canals thereof is used for multi-channel stereophonic sound recording. An acoustic cross-talk cancellation circuit is arranged to receive the microphone signals thereby to provide a binaural effect when reproduced through loudspeakers. There are circumstances, though, in which the use of further microphones remote from the dummy-head may he used as part of the recording process to provide a binaural effect. In such a situation it is necessary to know accurately the position of each remote microphone relative to the dummy head. There exist a variety of methods by which this position may be measured, such as using polar coordinates by utilising a theodolite and an optical range finder. Alternatively the Cartesian coordinates of the remote microphones and dummy head could be measured with respect to the boundaries of the room in which the recording is to take place, and then the azimuth angle, depression/elevation angle and the time-of-flight distance between the dummy-head and each remote microphone could be calculated. However such methods of measurement suffer from various shortcomings including the fact that distance measurements take a considerable time to carry out and are often very disruptive in a recording environment, especially if the remote microphones are deliberately moved to a different location during a recording session. Also the calculations based upon the measurements made are prone to cumulative errors, particularly for extreme positions where the angles subtended may be very small. Furthermore remote microphones may be physically difficult to access for measurement purposes due to being suspended several meters from the ground above an orchestra, for example. Another problem exists due to the fact that the time-of-flight between the remote microphones and the dummy-had is dependent on the speed of sound in air, which is itself dependent on both air temperature and humidity. JP-A-62 108 171 discloses a method of accurately calibrating the relative locations of a pair of echo sound receivers using a plurality of echo sound sources whose position is known from survey data. Information on sound source azimuth is known and is input as part of the position calculation. GB-A-2 115 150 discloses a surveillance system having a surveillance area comprising a plurality of cells, the system including a plurality of transducers responsive to the receipt of sounds processing means for providing signals representative of relative receipt times, and comparator means for comparing actual signals with those expected for sounds located in each of the cells to determine which cell the sound source is located in. It is an object of the present invention to provide a method and apparatus for positional determination in which the need for physically measuring angles and distances is avoided. Thus, according to a first aspect of the present invention there is provided a method of determining the position of a receiver relative to a given reference point according to claims 1 - 5. According to a further aspect of the present invention there is provided an apparatus for determining the position of a receiver relative to a given reference point according to claims 6 - 10. The present invention will now be described, by way of example only and with reference to the following drawings, of which: Figure 1 illustrates schematically an autocalibration system in accordance with the present invention; Figure 2 shows a schematic representation of signal transmission by the right loudspeaker of the autocalibration system of Figure 1; Figure 3 shows a schematic representation of signal transmission by the left loudspeaker of the autocalibration system of Figure 1; Figure 4 illustrates schematically how circles of propagation for the right loudspeaker are constructed; Figure 5 illustrates schematically how the position and angular displacement of a microphone is determined, and; Figure 6 shows a schematic representation of a second embodiment of the present invention. Referring firstly to Figure 1, a two-dimensional autocalibration system for a multi-microphone array in accordance with the present invention is illustrated in which all microphones and loudspeakers lie in the same plane. Two signal generators, in this case loudspeakers 2, 4 which are physically coupled via mounting bracket 6, are fed with transient pulses via their respective drive inputs 8, 10. It can be seen that the loudspeakers 2, 4 are placed one on either side of a dummy-head 12 such that the lateral centre-line 14 through the dummy-head 12 (i.e. through both ears from one side to the other) and the loudspeakers 2, 4 lie in the same plane. Three receivers, in this case microphones 16, 18, 20 whose positions in relation to the dummy-head 12 are to he determined are disposed in front of the head 12 and situated at unknown azimuth angles Θ 16 , Θ 18 and Θ 20 respectively to the centre-line 22 through the head 12 from its back to its front. Furthermore each microphone 16, 18, 20 lies at an unknown distance from the centre of the head 12 (the latter defined by the point of intersection of the two centre-lines 14 and 22); d 16 , d 18 and d 20 respectively. Each microphone 16, 18, 20 feeds into a respective preamplifier 24 and then into a respective high-precision analogue-to-digital (A/D) converter 26 after which the digitised signal is transferred into a local memory store 28 under the control of a signal processor 30 which communicates via control data bus 32. Each memory store 28 is capable of storing 200ms of data at a rate of 44.1 kbits per second. The control bus 32 also drives, in parallel, a pair of buffers 34 each of which is coupled to a respective digital-to-analogue (D/A) converter 36 and thence to a power amplifier 38. These power amplifiers 38 are, in turn, coupled to the respective drive inputs 8, 10 of the loudspeakers 2, 4. The autocalibration system functions as follows. Referring now also to Figure 2, a signal, here a transient pulse, is generated (in known manner) by the signal processor 30 and sent to the drive input 10 of the (right) loudspeaker 4 via the control bus 32 and the corresponding buffer 34, D/A 36 and amplifier 38. Simultaneously, the outputs of all the microphones 16, 18, 20 are transferred at a constant rate into their respective memory stores 28 via their respective preamplifiers 24 and D/As 26. These outputs are transferred to their respective memory stores 28 only for a pre-determined period, typically 100ms (or until the stores 28 are full), thus forming a temporary, time-domain record of their activity. One by one, the record of activity of each microphone 16, 18, 20 held within each respective memory store 28 is inspected by the signal processor 30 via data bus 32. This allows detection of the time location of the received transient pulse transmitted by the (right) loudspeaker 4 with respect to the beginning of the record (i.e. at the instant at which the pulse was propagated). Thus the time difference between the transmission of the pulse by the loudspeaker 4 and the time of arrival of the pulse at each microphone 16, 18, 20 can be determined by the signal processor 30. These transit times are known as the time-of-flight of the transit pulse from the loudspeaker 4 to each of the microphones 16, 18, 20. Each transit distance d1 16, d1 18 , d1 20 can be calculated directly from the corresponding time-of-flight measurement t 16 , t 18 , t 20 and the velocity of sound in air at room temperature and humidity (≈343 ms -1 ) using the relationship: d = vt where v = velocity of sound in air. Thus, for Figure 2, the three microphones 16, 18, 20 are located, respectively, at distances d1 16 , d1 18 , d1 20 from the loudspeaker 4, given by: d1 16 =vt 16 , d1 18 = vt 18 and d1 20 = vt 20 . Referring now to Figure 3 the above operation, described with reference to figure 2, is repeated using the (left) loudspeaker 8. This operation thus yields corresponding transit distances d2 16 , d2 18 , d2 20 for the microphones 16, 18 and 20 respectively. The location of each microphone 16, 18, 20 with respect to the dummy-head 12 can now be determined. Referring to Figure 4, if a circle having radius d1 16 is constructed around a centre which is the loudspeaker 4, then the circumference of this circle represents the location of the wavefront, emitted from the loudspeaker 4 at a time when the microphone 16 registered it. Similarly, the larger circle in figure 4, of radius d2 16 is constructed around a centre which is the loudspeaker 2. This circle corresponds to the "circle of propagation" from the loudspeaker 2 to the microphone 16. Hence, the microphone 16 must lie at the intersection of both circles, as shown. (It can be seen from Figure 4 that, by symmetry, the microphone could also lie at 16 1 , but it is known already that all three microphones 16, 18, 20 actually lie in front of the head 12 and so this "ghost" position can readily be discounted. In any event, this "ghost" can be removed simply by use of an additional loudspeaker set away from the plane of loudspeakers 2 and 4). Similar procedures are used to locate the positions of microphones 18 and 20. Referring now also to Figure 5 it is possible, from the transit distances calculated as described above, to determine the angular disposition, , of each microphone 16, 18, 20 with respect to a given reference point. In this example the given reference point is the centre of the dummy-head 12 defined by the points of intersection of the centre-lines 14 and 22. It is necessary to know the separation, x, of the loudspeakers along the centre line 14. Thus the distance of either speaker from the centre of the head 12 is x/2. Using the law of cosines d 16 can be derived by: d 16 = 1 2 ( d1 16 ) 2 +( d2 16 ) 2 - x 2 and thus  16 = cos -1 ( x 2 ) 2 +( d 16 ) 2 -( d1 16 ) 2 x ( d 16 ) Thus both the azimuth angle  16 and distance d 16 of the microphone 16 with respect to the dummy-head, as is required. It will be appreciated that, although only the azimuth angle  16 and distance d 16 for the microphone 16 have been described, this is for clarity only, and the same trigonometrical treatment is used to find  18 ,  20 and d 18 , d 20 as well as d1 18 , d2 18 and d1 20 , d2 20 . Referring now to figure 6, the case of determination of the position of the microphone 16 relative to a given reference point, here again the dummy-head 12, when the head 12 does not lie on the line 14 drawn between the two loudspeakers 2, 4 is illustrated. As in the example described herebefore, the separation, x, of the loudspeakers 2, 4 must be known and the position of the head 12 relative to a point, say the midway between the loudspeakers, also measured. In this figure, the head 12 is at distance w from the midpoint, parallel to line 14 joining the loudspeakers 2, 4 and at distance y from this midpoint in a direction perpendicular to line 14. As discussed before, the distances x, w and y are known from measurements and the distances d1 16 and d2 16 have been calculated from the time-of-flight measurements. Thus, from the cosine rule on the triangle ABM: d1 2 16 = d2 2 16 + x 2 -2 d2 16 x cosα and from triangle BMX : cosα = x + c d2 16 ∴ c = d2 2 16 - d1 2 16 - x 2 2 x and thus the intermediate value, c, may be derived. Now from triangle BMX: d2 2 16 = e 2 +( x + c ) 2 by Pythagoras which gives e = d2 2 16 -( x + c ) 2 and using Pythagoras on triangle MHY: d 1 16 = ( e - y ) 2 +( c + x 2 - w ) 2 thus d 1 16 may be derived. Now from triangle MHY: sin 16 = e - y d 1 16 thus  16 = sin -1 e - y d 1 16 It can be seen, from a consideration of the above examples that the distance and angular disposition of the microphone 16 relative to the head 12 may be determined from a knowledge of the time-of-flight measurements from each loudspeaker 2, 4 to the microphone and the distance measurements between the head 12 and the loudspeakers. From the foregoing it will be appreciated that the described system in accordance with the present invention automatically takes account of small changes in air velocity with changes in room temperature and humidity due to the fact that the times-of-flight are themselves measured acoustically. It will be apparent to those skilled in the art that although in the above example three microphones have been shown, there is a lower limit of only one such microphone being necessary and indeed more than three such microphones may readily be employed. Although the above example teaches using transient pulses transmitted by each loudspeaker in turn, any suitable signals may be used and there is no compulsion for their transmission to be from each microphone in turn. However, when transient pulses are employed, it is convenient for each microphone not to register subsequently received pulses after their first-received pulse from each loudspeaker has been registered. This obviates, for example, registration of stray reflectances from walls or the like. Those skilled in the art will realise that at least two loudspeakers are needed to implement the present invention. It will also be appreciated that, in the example described hereinbefore a planar system, in which all microphones and loudspeakers be in the same plane is illustrated. It may be convenient, however, for a three-dimensional system to be employed using three loudspeakers, such that not only can the distances and azimith angles of the microphones be derived, but also their angle of elevation (or depresion). Those skilled in the art all appreciate that the geometrical calculations provided hereabove can be extended to encompass the extra dimension. Those skilled in the art will appreciate that instead of measuring the separation of the loudspeakers and thus determining a midpoint from which the position of the reference point is measured, it would he equally efficacious to measure the position of the reference point relative to each loudspeaker directly. The geometrical calculations would then be altered, but clearly within the competence of one skilled in the art. It will also be understood that receivers could also be placed inside or around the dummy-head in the example described hereabove enabling calculation of the dummy head itself with respect to a known reference point.

Claims: A method of determining the position of a receiver (16) relative to a given reference point (12) comprising: a) transmitting acoustic signals from each of a plurality of acoustic signal generators (2, 4); b) receiving transmitted acoustic signals at the receiver; c) measuring the time-of-flight of the signals from each acoustic signal generator to the receiver; and, d) geometrically determining, from the time-of-flight measurements and the position of the given reference point relative to each acoustic signal generator, the distance and angular disposition of the receiver (16) relative to the given reference point, characterized in that the angular disposition of the receiver (16) is also determined geometrically from the time of flight measurements, and that the position of each receiver relative to said reference point is determined independently of the position of any other(s). A method according to Claim 1 wherein the acoustic signals transmitted from each of the acoustic signal generators are transient pulses. A method according to either Claim 1 or Claim 2 wherein acoustic signals are transmitted from each of the plurality of signal generators in turn. A method according to any one of the preceding claims including a plurality of acoustic receivers (16, 18, 20). A method according to Claim 4 wherein each of the receivers, after receiving the first of any signals transmitted by any given signal generator, does not register any subsequent signals received from the same signal generator until each of the plurality of signal generators has transmitted their signals. Apparatus for determining the position of a receiver (16) relative to a given reference point (12) according to Claim 1, comprising: a plurality of acoustic signal generators (2, 4) for transmitting acoustic signals therefrom; an acoustic signal receiver (16) for receiving the transmitted acoustic signals; and a signal processor (30) for measuring the time-of-flight of the acoustic signals from each acoustic signal generator to the receiver and geometrically determining, from the time-of-flight measurements and the position of the given reference point relative to each acoustic signal generator, the distance and angular disposition of the receiver relative to the given reference point. Apparatus according to Claim 6 wherein the acoustic signals transmitted from each of the acoustic signal generators are transient pulses. Apparatus according to Claim 6 or 7, including a plurality of receivers. Apparatus according to Claim 8 wherein the receivers are microphones. Apparatus according to any one of Claims 6-9 wherein the acoustic signal generators are loudspeakers.

Inventors: CLEMOW RICHARD, SIBBALD ALASTAIR, CLEMOW, RICHARD, SIBBALD, ALASTAIR
Applicants: CENTRAL RESEARCH LAB LTD, CENTRAL RESEARCH LABORATORIES LIMITED
Application year: 1994
Main classification: H04S3
IPC codes: H04R3, H04R5, H04S3